STEP export: ensure messages are immediately printed, and not stored in buffer.

The issue was mainly seen on Windows, in STEP export with msys2 build but also
with other builds when exported from Pcbnew dialog

common/reporter.cpp

@@ -130,6 +130,10 @@ REPORTER& CLI_REPORTER::Report( const wxString& aMsg, SEVERITY aSeverity )
     else
         wxFprintf( target, aMsg + wxS( "\n" ) );
 
+    // Needed  after wxPrintf (or printf) to be sure the message is immediately printed
+    // (i.e. not stored in some i/o buffer)
+    fflush( target );
+
     return *this;
 }